As computers have evolved, communication between computers has become desirable. In the past, several communications standards have been established to allow communications between computers and between computers and computer terminals. Recently, the CCITT (International Telegram and Telephone Consultive Committee) has proposed a Recommendation X.21 (herein incorporated by reference). The CCITT, an agency of the United Nations, issues recommendations rather than standards. While in the past, the CCITT recommendations have been almost identical to the standards of the Electrical Industries Association (EIA) standards such as the RS-232 communications standard, the X.21 Recommendation differs significantly from any of the present EIA standards. Briefly, the X.21 Recommendation provides an interface specification for data equipment connecting to a public data network using a digital synchronous format rather than an analog transmission. This X.21 Recommendation is directed to a public data network. The X.21 Recommendation includes the protocol for making and answering calls and for sending and receiving data after a connection is established.
A review of the communications standards is contained in a magazine article entitled "Welcome to the Standards Jungles" in the February, 1983 issue of BYTE Magazine, herein incorporated by reference.
One important aspect of implementing data communications is providing a mechanism to recognize certain bit patterns that are provided in serial bit streams from the network. Pending U.S. Patent Application Ser. No. 873,910 filed June 13, 1986 and assigned to the present assignee addresses this problem. In addition to requiring recognition of serial bit stream bit patterns, circuitry must be provided to interface to the communications network in accordance with the pre-described communications procedures of either the EIA standards or the CCITT Recommendations.
In the past, the central processing unit of a computer terminal included software that, when executed, interfaced with the communications network in accordance with the communications procedures. However, as the communications speed requirements increased, requiring the computer terminal processing unit to perform these communications tasks overly burdened the computer terminal. An alternative approach is the provision of a second processing unit in the computer terminal to perform the task of communicating with the network. However, the problem of inter-communications between the computer terminal processing unit and the communications processing unit exists.
It is an object of the present invention to provide a terminal communications circuit that includes a program for execution in the computer terminal processing unit but further provides circuitry to perform much of the network communications tasks resulting in communications control remaining with the computer terminal processing unit while not burdening the computer terminal processing unit with the overhead requirement of the communications procedures.